High strain point glasses

ABSTRACT

A family of titania lanthana aluminosilicate glasses, and products such as an electronic device having a poly-silicon coating on such glass as a substrate, are disclosed. The glasses have a strain point in excess of 780° C., a coefficient of thermal expansion of 20-60x10-7/° C., a Young&#39;s modulus of greater than 12 Mpsi and are chemically durable.

FIELD OF THE INVENTION

The invention relates to (TiO₂, Ta₂O₅)—La₂O₃—Al₂O₃—SiO₂ glasses that arecharacterized by a high strain point, a low coefficient of thermalexpansion and a capability of being produced as sheet glass byconventional methods.

BACKGROUND OF THE INVENTION

Glass substrates for electronic devices have been limited to use andprocessing temperatures not over about 600-650° C. For highertemperatures, the only available transparent materials have been fusedsilica or a class of glass-ceramics. These materials are difficult, andtherefore expensive, to produce.

Fused silica provides a high strain point (typically>1000° C.) andexcellent thermal stability. However, it is difficult to produce andfabricate. Also, it has a low (5×10⁻⁷/° C.) coefficient of thermalexpansion (CTE) that is not compatible with such electronic materials assilicon.

Transparent, spinel glass-ceramics also provide high strain points(typically>900° C.), and provide a better expansion match with silicon(25-40×10⁻⁷/° C.). However, ceramming these materials adds to theircost. Also, their predecessor glasses tend to be very fluid at theirliquidus temperatures. This poses a challenge to forming precision sheetglass; as well as other glass forms.

A need exists, then, for a glass that (1) has a high strain point (>780°C.), (2) does not require costly heat treatments after fabrication, (3)that can be formed at a viscosity greater than 10³ poises, and (4) canbe melted in a conventional melting unit. In addition, the glass must betransparent to visible radiation and be chemically durable. Theseseveral qualities are needed in glasses for production of such variedproducts as flat panel displays, photovoltaic cells, and tubing andfiber applications that require stability at high temperatures.

Flat panel displays employ sheet glass that necessarily is transparentat visible wavelengths as well as into the ultra violet. It is alsonecessary that the glass sheet be adapted to production of a siliconlayer on the glass surface. Initially, the silicon layer applied wasamorphous silicon (a-Si). Fabrication of such devices requiredtemperatures no greater than 350° C. Suitable glasses were readilyavailable for use under these conditions.

The evolution from a-Si to poly-Si (polycrystalline silicon) as acoating material has presented a major challenge to use of a glasssubstrate. Poly-Si coatings require much higher processing temperatures,in the range of 600-1000° C.

One available substrate material is fused silica. This material has ahigh strain point of about 1000° C. and excellent thermal stability.However, it has a low CTE that is markedly lower than poly-Si.Furthermore, the ability to fabricate this material is limited, and, atbest, very expensive.

Another potential candidate is a family of transparent, spinelglass-ceramics. These materials have the required high strain pointof >780° C. They are also reasonably well matched to poly-Si in CTE.However, the additional ceramming process adds significantly to the costof production. Perhaps more important is the fact that the precursorglasses of these glass-ceramics are very fluid at their liquidustemperatures. This presents a serious challenge to formation ofprecision sheet glass.

Glasses available from Corning Incorporated under Codes 1737 and 2000can be used for some low temperature applications on the order of600-650° C. This glass is an aluminosilicate glass that contains amixture of divalent metal oxides and is essentially free of alkali metaloxides. Even this glass must be subjected to special thermal treatmentto avoid shrinkage or compaction during poly-Si deposition.

The efficient production of high quality, poly-Si, thin films requiresthermal annealing at temperatures in the 800-900° C. range. This highertemperature anneal enables shortening the annealing time. It alsoresults in excellent uniformity at the coating-substrate interface, andmore stable performance of a device over time. Except for the fusedsilica and glass-ceramic substrates mentioned above, there has not beena suitable, substrate material available.

A primary purpose of the present invention is to provide a glass thathas properties suited to production of a poly-Si coating on its surface.

Another purpose is to produce a glass having a sufficiently high strainpoint to permit processing at 800-900° C.

A further purpose is to provide a glass that can be melted and formed byconventional procedures employed in producing sheet glass, and that canprovide a substrate for application of a high quality, poly-Si film.

A still further purpose is to provide an electronic device, inparticular, a flat panel display, embodying a sheet glass substrate,produced in a conventional manner, and having a high-quality, poly-Si,thin film on its surface.

Another purpose is to provide a novel glass family consistingessentially of (TiO₂ and/or Ta₂O₅), La₂O₃, Al₂O₃ and SiO₂, andoptionally containing selected oxides including Y₂O₃, ZrO₂, HfO₂, SnO₂,GeO₂, Ga₂O₃, Sb₂O₃, B₂O₃ and/or P₂O₅.

SUMMARY OF THE INVENTION

The invention resides in part in a family of titania lanthanaaluminosilicate glasses having a strain point in excess of 780° C., acoefficient of thermal expansion of 20-60×10⁻⁷/° C., a Young's modulusgreater than 8.28×10⁴ MPa (12 Mpsi), and a weight loss of less than onemg/cm² in BHF (buffered HF). Titania, or equivalently tantalum oxide, isan essential constituent which serves to lower the CTE of the glass to avalue compatible with poly-silicon. These oxides also act as fluxes,steepen the viscosity curve, and increase strain point.

The invention further resides in an electronic device having apoly-silicon film on a transparent, glass substrate, the substrate beinga titania lanthana aluminosilicate glass having a strain point in excessof 780° C., a coefficient of thermal expansion of 20-60×10⁻⁷/° C., aYoung's modulus greater than 8.28×10⁴ MPa (12 Mpsi), and a loss of lessthan one mg/cm² in BHF.

DESCRIPTION OF THE INVENTION

This invention is based on our discovery of a family of titania lanthanais aluminosilicate glasses. These glasses possess unique propertiesparticularly suited to production of electronic devices having a film ofpoly-silicon on the glass surface. In particular, the glasses have astrain point in excess of 780° C. and a coefficient of thermal expansion(CTE) of 20-60×10⁻⁷/° C., preferably in the range of 20-40×10⁻⁷/° C. Theglasses also have a high Young's modulus, greater than 8.28×10⁴ MPa (12mpsi), and are chemically durable, especially in buffered HF (BHF). Thetitania can be partially or completely replaced by tantalum oxide withno detrimental effects on CTE or strain point.

The buffered HF test determines the weight loss in mg/cm² and sampleappearance of a glass sample after immersion in a buffered hydrofluoricacid solution for five (5) minutes. The solution consists of one volumeof 50% by weight HF and ten volumes of 40% by weight ammonium fluoride(NH₄F). The solution is maintained at 30° C. for the test.

As pointed out earlier, there is an available commercial glass that canbe melted in conventional manner and that can have a poly-silicon filmdeposited on the glass surface. To accomplish this requires specialtreatment of the glass, and the operation must be carried out attemperatures not over 650° C.

The efficient production of devices having a high quality poly-siliconfilm requires thermal annealing of the film at temperatures in the rangeof 800-900° C. However, a conventionally produced glass, capable of useat such temperatures, has not been available. The present inventionprovides such glasses.

The glasses of the present invention are members of a titania lanthanaaluminosilicate (TiO₂—La₂O₃—Al₂O₃—SiO₂) glass family. They arecharacterized by a very high strain point (greater than 780° C.) and aCTE closely matching that of silicon (25-40×10⁻⁷/° C.). As alreadystated, The titania can be partially or completely replaced by tantalumoxide with no detrimental effects on CTE or strain point.

Broadly stated, the present glasses have compositions falling within thefollowing ranges, expressed in mole % as calculated from the glass batchon an oxide basis:

SiO₂ 40-90% TiO₂ 0-20% Al₂O₃  5-35% Ta₂O₅ 0-10% La₂O₃  2-30% (TiO₂ +Ta₂O₅₎ 0.5-20%   RO 0-10%

where R is Mg, Ca, Sr, Ba and/or Zn. In the range of 40-60% SiO₂,lanthanum oxide can be partially or completely replaced by yttriumoxide, lowering the density and CTE while maintaining or even raisingthe strain point. Other optional constituents which are compatible withthe inventive glasses include ZrO₂, HfO₂, SnO₂, GeO₂, and/or Ga₂O₃.Additions of up to 10% on a molar basis of each of these oxides and/orup to 3% on a molar basis of Sb₂O₃, WO₃, B₂O₃ and/or P₂O₅ can be madewithout lowering the strain point below 780° C. Beyond these levels,glasses either become unstable or their properties fall below that ofthe inventive glass compositions.

These glasses have the following characteristic properties:

Strain Point >780° C. CTE 20-60 × 10⁻⁷/° C. Young's modulus >8.78 × 10⁴MPa Durability in BHF <1 mg/cm²

A preferred embodiment has compositions within these ranges, again inmol % on an oxide basis, as calculated from the glass batch:

SiO₂ 70-84% Al₂O₃  6-18% La₂O₃  2-15% TiO₂  0-8% Ta₂O₅  0-8% (TiO₂ +Ta₂O₅) 0.5-10%  RO    <3%

These preferred glasses are characterized by the following properties:

Strain Point 840-900° C. CTE 20-40 × 10⁻⁷/° C. Young's modulus 8.78 ×10⁴ MPa Durability in BHF ˜0.5 mg/cm² weight loss

Clear, single-phase glasses that were well-fined were obtained bymelting 4.54 kg (ten lb.) crucible melts of compositions within thepreferred ranges at a temperature of about 1650° C. for times not over20 hours. It is essential that La₂O₃ be present in the indicated amountto obtain such melts; also that the total content of La₂O₃ plus TiO₂ andTa₂O₅, if present, be not greater than the Al₂O₃. The presence of TiO₂and/or Ta₂O₅ is necessary for a maximum strain point and a minimum CTE.Preferably, the contents are such that the ratio of La₂O₃ to TiO₂+Ta₂O₅is between 1 and 4.

The position of the UV edge in glasses containing TiO₂ is controlled byincluding As₂O₃ or CeO₂ in the composition. The oxides buffer againstreduction of Ti⁺⁴ to Ti⁺³. They also enhance fining, as does employinghalides, e.g. AlCl₃, AlF₃, and/or AlBr₃, instead of oxides in the glassbatch to be melted.

TABLE I, below, sets forth several compositions, in mol % on an oxidebasis, illustrative of the invention. Also shown are characteristicproperties as measured on the glass produced.

TABLE I Series 882 882 889 882 889 889 889 889 889 Code COJ COT CWB CVNDAQ DAT CXZ CXM CVL SiO₂ 40 55 70 70 76 82 82 84 86 Al₂O₃ 30 15 18.8 1715 11.25 12 10 8 La₂O₃ 25 25 5.6 13 4.5 3.4 4 5 5 TiO₂ 5 5.6 4.5 3.4 2 1Ta₂O₅ 5 1 Softening Pt. 956 984 1050 1015 1110 (° C.) Anneal Pt. 826 846839 910 939 948 928 958 (° C.) Strain Pt. 790 805 802 834 862 882 854880 (° C.) CTE_(RT-300) 62.1 64.6 32.0 54 28.4 23.3 22.7 27.9 27.5(×10⁻⁷/° C.) Density 4.26 4.65 2.91 3.4 2.78 2.64 2.68 2.76 2.73 (g/cm₂)Young's modulus ×10⁴ MPa 8.9 8.4 8.35 8.2 Durability 0.49 0.5 (mg/cm₂)

The strain points are somewhat lower than fused silica or the spinelglass-ceramics. However, they are substantially higher than available,conventionally melted glasses, and quite adequate for their intendedpurpose. The slightly lower strain point is more than offset by the useof conventional melting and processing procedures and other favorableproperties, such as matching expansion and favorable viscosity at theglass liquidus, thus, a glass having the composition of Example 7 inTABLE I, in addition to having a strain point of 882° C. and a CTE of22.7×10⁻⁷/° C., has a liquidus viscosity of 7000 poises.

It has been observed that a high ratio of La₂O₃ to TiO₂ provides greaterglass stability during forming of glass by a down-draw process. Forexample, the glass of Example 4 in TABLE I can be drawn as thick-walledtubing of a precision nature. The draw was at a temperature over 150° C.below the liquidus temperature of the glass. Tubes and rods of thisglass have also been drawn in a commercial draw tower to provide ahigh-strength fiber.

We claim:
 1. A titania lanthana aluminosilicate glass having a strainpoint in excess of 780° C., a coefficient of thermal expansion (0-300°C.) of 20-60×10⁻⁷/° C., a Young's modulus greater than 12 Mpsi, and aweight loss less than one mg/cm² in BHF wherein the titania can bepartially or completely replaced by tantalum oxide.
 2. The glass ofclaim 1 consisting essentially of, expressed in mol percent andcalculated from the glass batch on an oxide basis, 40-90% SiO₂, 5-35%Al₂O₃, 2-30% La₂O₃, and 0.5-20% (TiO₂+Ta₂O₅).
 3. The glass of claim 1consisting essentially of, expressed and calculated from the glass batchon an oxide basis, 40-90% SiO₂, 5-35% Al₂O₃, 2-30% La₂O₃, 0.5-20%(TiO₂+Ta₂O₅), and 0-10% ZrO₂, HfO₂, SnO₂, GeO₂, and/or Ga₂O₃.
 4. Theglass of claim 1 consisting essentially of, expressed in mol percent andcalculated from the glass batch on an oxide basis, 40-90% SiO₂, 5-35%Al₂O₃, 2-30% La₂O₃, 0.5-20% (TiO₂+Ta₂O₅), and 0-3% Sb₂O₃, WO₃, B₂O₃and/or P₂O₅.
 5. The glass of claim 1 consisting essentially of,expressed in mol percent and calculated from the glass batch on an oxidebasis, 40-90% SiO₂, 5-35% Al₂O₃, 2-30% La₂O₃, 0.5-20% (TiO₂+Ta₂O₅), and0-10% of one or more of the divalent metal oxide MgO, CaO, SrO, BaO andZnO.
 6. A lanthana aluminosilicate glass in accordance with claim 1consisting essentially of, expressed in mol percent and calculated fromthe glass batch on an oxide basis, 70-84% SiO₂, 6-18% Al₂O₃, 2-15%La₂O₃, and 1-8% (TiO₂+Ta₂O₅).
 7. The glass of claim 1 consistingessentially of, expressed in mol percent and calculated from the glassbatch on an oxide basis, 40-90% SiO₂, 5-35% Al₂O₃, 2-30% La₂O₃, 0.5-20%(TiO₂+Ta₂O₅), and less than 3% of one or more of MgO, CaO, SrO, BaO,and/or ZnO.
 8. The glass of claim 5 having a strain point in the rangeof 840-900° C. and a CTE in the range of 20-40×10⁻⁷/° C.
 9. Anelectronic device comprising a poly-silicon film on a transparent, glasssubstrate, the substrate being a titania lanthana aluminosilicate glasshaving a strain point in excess of 780° C., a coefficient of thermalexpansion in the range of 20-60×10⁻⁷/° C., a Young's modulus greaterthan 12 Mpsi and a weight loss not over one mg/cm² in BHF wherein thetitania can be partially or completely replaced by tantalum oxide. 10.An electronic device in accordance with claim 9, wherein the substrateglass consists essentially of, expressed in mol percent and calculatedfrom the glass batch on an oxide basis, 40-90% SiO₂, 5-35% Al₂O₃, 2-30%La₂O₃, and 0.5-20% (TiO₂+Ta₂O₅).
 11. An electronic device in accordancewith claim 9 wherein the substrate glass consists essentially of,expressed in mol percent and calculated from the glass batch on an oxidebasis, 40-90% SiO₂, 5-35% Al₂O₃, 2-30% La₂O₃, 0.5-20% (TiO₂+Ta₂O₅), and0-10% ZrO₂, HfO₂, SnO₂, GeO₂, and/or Ga₂O₃.
 12. An electronic device inaccordance with claim 9 wherein the substrate glass consists essentiallyof, expressed in mol percent and calculated from the glass batch on anoxide basis, 40-90% SiO₂, 5-35% Al₂O₃, 2-30% La₂O₃, 0.5-20% (TiO₂+Ta₂O₅)and 0-3% Sb₂O₃, WO₃, B₂O₃ and/or P₂O₅.
 13. An electronic device inaccordance with claim 9 wherein the substrate glass consists essentiallyof, expressed in mol percent and calculated from the glass batch on anoxide basis, 40-90% SiO₂, 5-35% Al₂O₃, 2-30% La₂O₃, 0.5-20% (TiO₂+Ta₂O₅)and 0-10% of one or more of the divalent metal oxides MgO, CaO, SrO, BaOand ZnO.
 14. An electronic device in accordance with claim 9 wherein theglass substrate is a titania lanthana aluminosilicate glass consistingessentially of 70-84 mol % SiO₂, 6-18% Al₂O₃, 2-15% La₂O₃, and 1-8%(TiO₂+Ta₂O₅) and having a strain point in the range of 840-900° C. and aCTE in the range of 20-40×10⁻⁷/° C. wherein the titania can be partiallyor completely replaced by tantalum oxide.